Modular articulated-wing aircraft

ABSTRACT

Systems and/or methods for forming a multiple-articulated flying system (skybase) having a high aspect ratio wing platform, operable to loiter over an area of interest at a high altitude are provided. In certain exemplary embodiments, autonomous modular flyers join together in a wingtip-to-wingtip manner. Such modular flyers may derive their power from insolation. The autonomous flyers may include sensors which operate individually, or collectively after a skybase is formed. The skybase preferably may be aggregated, disaggregated, and/or re-aggregated as called for by the prevailing conditions. Thus, it may be possible to provide a “forever-on-station” aircraft.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Application Ser. No. 60/697,497,filed Jul. 7, 2005, the entire contents of which are incorporated hereinby reference.

FIELD

The exemplary embodiments herein relate to high altitude aircraftflights. More particularly, the exemplary embodiments herein provide ameans of attaining a very high aspect ratio wing platform with a lowerstructural weight than is achievable with existing designs. This lowerweight allows both a higher ceiling and longer endurance at higheraltitudes than would otherwise be possible. Additionally, the modularnature of the exemplary embodiments provides greatly increasedoperational flexibility and system robustness.

BACKGROUND AND SUMMARY

For many military and commercial missions it is desirable to fly anaircraft at very high altitude, ideally with the capability to stay onstation indefinitely. The missions that benefit from this capability arethose that take advantage of the long line-of-sight to the horizonenjoyed by such a high altitude platform. Missions with both militaryand commercial utility include surveillance and communicationsconnectivity. A high altitude, long endurance aircraft also hasapplication to the military signals intelligence mission. Finally, ahigh altitude, long endurance aircraft has application to spaceexploration, inasmuch as such a vehicle can be flown in any type ofplanetary atmosphere. An aerodynamic design that can operate at the verytop of the Earth's atmosphere is naturally well-suited to flying at alower altitude on a world with low atmospheric density at the planetarysurface.

Alternatives to high altitude, long endurance aircraft include orbitingsatellites and those aircraft that can achieve high altitude but withoutthe capability for long endurance at that altitude. Both of thesealternatives have operational and cost disadvantages. Satellites areexpensive to launch and operate and, except in the case of geostationarysatellites, cannot loiter over a point on the ground. Consequently,large constellations of satellites are required for global coverage orto ensure high revisit rates (i.e. short gaps in coverage) with respectto a ground target or ground station. Geostationary satellites remainfixed with respect to a location on the ground only when launched intoan orbital slot above the equator, which drastically limits groundcoverage, especially at high latitudes. Finally, aircraft without longendurance at high altitudes have inefficiencies of operation, owing tothe need to cycle such vehicles back and forth from a launch base to amission station (e.g. the locus of the surveillance, communication, orother mission activities in question). For example, at any given timeone aircraft might be flying to relieve a second aircraft on-station,while a third is flying back to the launch base from the station, and afourth is at the launch base being prepared for takeoff. The requiredfleet size, and thus the overall cost, increases with the distance ofthe mission station from the launch base. Furthermore, along the entireflight path of the cycling aircraft, the operation becomes subject tothe vagaries of nature (e.g. storms) and, in the military case, enemyaction, etc.

As a result of the cost and operational disadvantages of alternatives, aviable high altitude long endurance aircraft has become something of aholy grail for aircraft designers. Furthermore, for reasons ofoperational responsiveness, it is desirable that such an aircraft berapidly deployable to a distant operating location without being impededby adverse weather conditions. Satisfying these requirements with onedesign is an extremely difficult technical challenge.

For reasons of less-than-perfect subsystem reliability, no high altitudelong endurance aircraft that could operate on-station indefinitely hasheretofore been conceived. In the past, it has been recognized that itis possible to design aircraft, the endurance limits of which are notbounded by the supply and consumption of onboard fuel. Such an aircraftcould maintain a mission station at an altitude for perhaps severalmonths, until subsystem failures forced a return to base. There arethree general cases: nuclear propulsion using an onboard nuclear fissionreactor, power beamed to the aircraft from the ground (e.g., usingmicrowaves or laser energy), and solar-electric propulsion.

The United States explored nuclear-powered aircraft in the 1950s, butthe effort that involved a modified Convair B-36 Peacemaker test-bedaircraft and ground-based test articles was terminated. It is highlyunlikely that contemporary environmental awareness and politicalsensitivities would allow a similar concept to be pursued today.

Small remote-controlled aircraft that are powered by means of energybeamed from a ground site have been designed and, in some cases, flown.Effectiveness is limited by very poor efficiencies when distance fromthe ground site becomes large, as a consequence of beam spreading andthe resulting reduced energy flux received by the aircraft. Furthermore,if beam spreading is minimized by resorting to higher frequencies ofenergy transmission, flux is improved at the cost of increasedenvironmental risk (e.g. birds and other aircraft may fly through thebeam at intermediate altitudes). The practical result of theselimitations is that the beam-powered aircraft is virtually tetheredclose to its source of power, which is operationally undesirable in mostcases.

Solar-electric propulsion is the third pathway to effectively unlimitedflight and, in fact, full-scale unmanned and manned solar-electricairplanes have been flown. Examples include the Aerovironment Pathfinderand Helios. Reliance on solar flux causes solar-electric aircraftdesigns to have very low propulsive power, which in turn places apremium on aerodynamic and structural design. Furthermore, such aircraftare best operated at very high altitude, ideally more than 50,000 feetabove sea level, to ensure that clouds do not reduce received solar fluxand to minimize the chance of encountering headwinds.

As a result of these considerations, current efforts to achieve a“forever on-station” high altitude aircraft have largely focused onsolar-electric aircraft. There are two types of aircraft underconsideration: heavier-than-air aircraft (e.g. airplanes) andlighter-than-air aircraft (e.g. airships). Airships derive their liftfrom aerostatic means (e.g. from a buoyant force provided by a liftinggas such as helium) rather than from aerodynamic forces acting on awing. A solar-electric airship currently under development is theLockheed-Martin High Altitude Airship.

In both airplane and airship cases, the combination of low power (whichis due to the limits of solar flux) and high altitude results in theneed for very large, lightweight structures. In the airplane case, wingloading (i.e. the ratio of airplane weight to wing area) must be verylow. In the airship case, hull fabric weight per surface area must bevery low. Consequently, both airplanes and airships will be relativelyfragile. Additionally, airspeeds of both types of vehicle will be verylow due to the low power that is available. These aircraft areconsequently at risk of catastrophic structural failure or being blownuncontrollably downwind, as a result of gusts or high windsrespectively, while climbing or descending through the lower atmosphereor while being launched.

The most efficient aerodynamic configuration in terms of lift-to-dragratio for a high altitude solar-electric aircraft is that of a highaspect ratio unswept flying wing, where aspect ratio is defined as thesquare of wing span divided by reference wing area. “Flying wing” refersto an airplane that is comprised of a wing alone, without fuselage orempennage. This was, in fact, the configuration of the AerovironmentPathfinder and Helios aircraft. The primary aerodynamic disadvantage ofsuch a configuration is that stability and control are inherently poor,especially in the longitudinal or pitch sense, since with no tailsurfaces there can be no significant tail moment arm. The primarystructural disadvantage of the lightweight, high aspect ratio flyingwing configuration is that there can be little resistance to span-wisebending and little torsional stiffness (i.e. resistance to wingtwisting). In particular, in the solar-electric case, there is no fuelcarried in the wing, the weight of which would serve to react againstthe first wing bending moment. If payload is not distributed across thespan of the wing (i.e. span-loaded) but is instead concentrated at thecenterline of the vehicle, the problem of span-wise bending isaggravated. Finally, these aerodynamic and structural difficulties cancombine in the form of aero-structural interactions—for example, theaircraft can develop wing flapping and twisting oscillations that causeuncontrollable and potentially divergent oscillations in flight path.This sequence of events was, in fact, the proximate cause of thein-flight breakup of the Aerovironment Helios over the Pacific Ocean in2003.

Returning to the airship case, the lightweight fabrics required for highaltitude airship flight are problematic. For reasons of weight, highaltitude airships must be of non-rigid design, where hoop stresses andhull bending moments are carried by the hull fabric alone. Such fabricsmust also resist tearing, resist ultraviolet radiation, and be veryimpermeable to helium. Historically, hull structural failure of airshipsoperating at low altitude has been a recurring difficulty, and therequirement for lightweight fabrics at high altitude makes mattersworse. Finally, to carry a reasonable payload, the high altitude airshipmust be extraordinarily large, on the order of 500 feet in length ormore. This limits basing opportunities and introduces ground handlingdifficulties.

The exemplary embodiments described herein incorporate the premise thatthe technological and programmatic risks associated with high altitudeairships are greater than those of high altitude airplanes, and proposesa solution for the aero-structural limitations of high aspect ratioflying wing airplanes. This solution entails subdivision of the winginto autonomous modular units that can join together in-flight,wingtip-to-wingtip, forming a single, multiple-articulated flyingsurface of great aerodynamic efficiency. A preferred embodiment includesa modular articulated-wing aircraft as above, with a solar-electricpower system to provide motive force and satisfy mission system andhousekeeping electrical demands.

There are in principle two ways of arranging low aspect ratio wingelements so as to approximate the aerodynamic efficiency of a higheraspect ratio wing. The first is, as above, to join the wing elements atthe wingtips, creating an actual continuous wing surface. The secondapproach is to form a virtual wing, where wing elements are arranged ina chevron as seen from above, akin to the arrangement of a flock ofgeese flying in formation. In this latter case, aerodynamic benefitsaccrue from trailing wing elements being positioned precisely in theupwash of the element in front—in effect, the wing element is hitching aride on the preceding element. In theory, the virtual wing approach canlead to impressive gains in aerodynamic efficiency, and since the wingelements are physically isolated there is no difficulty with wingbending. However, there are practical difficulties. The relativepositions of wing elements must be very precisely controlled—thevorticity of airflow behind a wing means that a slight shift in lateralpositioning can result in a wing element being in the downwash ratherthan upwash of the preceding element. There must be constant rotation inthe positions of elements in the virtual wing, as the lead element getsno “free ride” and must periodically fall back, as does the lead goosein a flight of geese. Finally, and perhaps most seriously, aerodynamicmodeling of such a virtual wing is difficult and the net aerodynamicbenefits of the configuration currently are speculative.

The concept of aircraft joined at the wingtips to improve aerodynamicperformance is not new. However, the prior art is restricted to aircraftof unequal sizes joined with the advantages of improving range andendurance rather than identically-sized aircraft joined with theadvantage of attaining high altitude. Generally, small “hitchhiker”aircraft attach themselves to the wingtips of a much larger “mothership”aircraft (e.g. fighters attached to the wingtips of a bomber), enablingthe hitchhikers to cover long distances that would otherwise be beyondtheir capability. Meanwhile, thanks to the aerodynamic advantage of aneffectively higher aspect ratio wing, the mothership incurs little or nofuel consumption penalty.

The United States Air Force conducted flight tests ofhitchhiker-mothership compound aircraft beginning in 1949. The objectivewas to demonstrate the capability for intercontinental bombers to beescorted for thousands of miles to their targets and back, and this wasonly possible if the fighters were carried or assisted by the bombers insome fashion. From 1949 to 1950, flight tests of a wingtip-linkedDouglas C-47A transport and a Culver Q-14B trainer were conducted. Thesetests were promising, and were followed by tests of a Boeing B-29Superfortress bomber linked at the wingtips to two Republic F-84 jetfighters in a project designated “Tip Tow.” Unfortunately the B-29 andone of the F-84s were lost with all souls in 1953. An automatic flightcontrol system whose purpose was to control flapping angle failed tofunction as expected, and the doomed F-84 rotated about the wingtipconnection, impacting the wing of the B-29. Flapping angle is defined asthe angle between the wings of two joined aircraft in the lateraldirection.

Another Air Force tip-docking project designated “Tom Tom” was conductedfrom 1952 to 1953. The Tom Tom project flight tested a Convair B-36Peacemaker bomber attached at the wingtips to two F-84 fighters. On atest flight in late 1953, an uncontrollable oscillation developedbetween the B-36 and one of the F-84s, and the B-36 suffered majordamage to its wing. The F-84 returned to base with a large section ofthe B-36's wing structure still attached to its wingtip.

As a result of these difficulties, Projects Tip Tow and Tom Tom werecancelled, and the Air Force ceased further experimentation withtip-docking compound aircraft concepts. The technology of the time wasdeficient in a number of areas. It was difficult or impossible toanalytically predict complex flow fields and the interactions offlexible, linked aero-structures. It was an enormous challenge to designthe automatic flight control systems that were necessary for tip-linkedoperations. Note that the hitchhiker-mothership type of compoundaircraft has inherent difficulties that are not a feature of compoundaircraft comprising multiple, small, equal-sized flight elements.Specifically, the mothership is large and heavy relative to thehitchhikers, and consequently the hitchhikers contend with very strongtrailing wingtip vortices generated by the mothership. These vorticesbecome a hazard during docking or undocking maneuvers.

In 2002, a doctoral dissertation by S. A. Magill titled “CompoundAircraft Transport Study: Wingtip-Docking Compared to Formation Flight”was published by Virginia Polytechnic Institute. This document outlineda technical investigation of the hitchhiker-mothership type of compoundaircraft in tip-docked and formation flight modes. The latter modeinvolves the creation of a virtual wing in chevron as discussed in thepreceding text. The document did not consider the tip-docking ofmultiple, equal-sized aircraft. It did not address the pursuit of anytype of compound aircraft design for the purpose of improving aircraftceiling or performance at high altitude.

Thus, it will be appreciated that there is a need in the art to overcomeone or more of these and/or other disadvantages. It also will beappreciated that there is a need in the art to provide a viable highaltitude long endurance aircraft.

In certain exemplary embodiments, an autonomous modular flyer operableto loiter over an area of interest at a first high altitude is provided.Such flyers may comprise an airborne object having two wings, with eachwing having a wingtip, and the wingtips being operably joinable to atleast one other autonomous modular flyer's wingtips to form anaggregation when a first predetermined condition is met, and beingoperably disaggregable from the at least one other autonomous modularflyer's wingtips when a second predetermined condition is met. Theaggregation may form a multiple-articulated flying system having a highaspect ratio wing platform, operable to loiter over the area of interestat an altitude at least as high as the first high altitude.

Autonomous modular flyers and/or aggregations thereof may be furtheroperable to match their airspeed to a prevailing headwind and/or to makelarge orbits. Autonomous modular flyers and/or aggregations thereof mayhave an altitude ceiling in Earth's stratosphere and/or structuralrobustness in Earth's troposphere. The autonomous modular flyer mayfurther comprise a wingtip hinge on at least one wingtip allowing twooperably joined modular flyers to flap about the wingtip hinge withrespect to each other.

Aggregations of larger numbers of modular flyers may occur atsequentially higher altitudes. A second predetermined condition mayinclude one or more of: a loading event above a given load threshold, agust above a gust threshold, a turn of the multiple-articulated flyingsystem, a span shear above a span shear threshold, an instruction for atleast one of the modular flyers to undertake a remote surveillanceactivity, and an instruction for at least one of the modular flyers tomove closer to the area of interest. The multiple-articulated flyingsurface of claim 1 may be operable to reaggregate based at least on athird predetermined condition. That third predetermined condition mayinclude one or more of: a second predetermined condition that previouslywas met no longer is met, and at least one modular flyer beingdestroyed, recalled, and/or no longer functional.

Insolation circuitry may power each modular element and/or themultiple-articulated flying system, and the insolation circuitry maycomprise a photovoltaic array, an electronic controller to condition andmanage the power, and an electrical energy storage mechanism. A flightcontroller operable to calculate an equilibrium ceiling altitude and toinstruct the autonomous modular flyer to climb or descend to theequilibrium ceiling altitude may be included in modular flyers.

Certain exemplary embodiments provide a method of forming amultiple-articulated flying system having a high aspect ratio wingplatform, operable to loiter over an area of interest at a highaltitude. Such methods may comprise providing at least two autonomousmodular flyers, with each having two wings with wingtips thereon. Thewingtips of the at least two autonomous modular flyers may be joinedwhen a first predetermined condition is met.

Such methods may further comprise calculating an equilibrium ceilingaltitude for the autonomous modular flyer, and altering the autonomousmodular flyer's altitude to the equilibrium ceiling altitude. Also, anequilibrium ceiling altitude for the multiple-articulated flying systemmay be calculated, and the multiple-articulated flying system's altitudemay be altered to match the equilibrium ceiling altitude.

Also, data related to the area of interest may be sensed by anindividual modular flyer. When a multiple-articulated flying system isformed, data may be shared between sensors of modular flyers and/orusing individual sensors of modular flyers as elements in a sensorarray.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages will be better and morecompletely understood by reference to the following detailed descriptionof exemplary illustrative embodiments in conjunction with the drawings,of which:

FIG. 1 shows an exemplary multiple-articulated flying system having ahigh aspect ratio wing platform, and an enlarged view of an exemplaryflyer comprising such a system;

FIG. 2 is an exemplary lifecycle of a skybase;

FIG. 3 shows an exemplary deployment process;

FIGS. 4A-4C show an exemplary assembly process;

FIGS. 5A-5C show an exemplary reaggregation process;

FIG. 6 shows solar flux in watts per square meter as a function ofseason and latitude;

FIGS. 7A-7F show skybase equilibrium ceilings as a function of thenumber of connected flyers at various latitudes and at various times ofthe year;

FIG. 8A is a plot of mean wind speed as a function of altitude andlatitude for the case of a northern hemisphere winter;

FIG. 8B is superimposes a representation of estimated skybaseperformance on the mean wind plot from FIG. 8A;

FIG. 9 shows skybase airspeed superimposed on winds aloft near Baghdad,Iraq, observed between 1958 and 1990;

FIG. 10 sets forth a perspective view of an element of the modulararticulated-wing aircraft in a preferred embodiment (e.g. one of theflyers comprising a skybase);

FIG. 11 depicts an alternative exemplary embodiment of the skybaseflyer, showing mechanical and aerodynamic aids to tip-docking;

FIG. 12 shows an exemplary arrangement of two flyers packaged into astandard 40-foot shipping container;

FIG. 13 illustrates a preferred embodiment of the skybase connection andarticulation mechanism;

FIG. 14 shows the restoring moment acting about the flapping axis of twoconnected flyers where one flyer is at the outside of the skybase; and,

FIG. 15 shows actual outboard wingtip effects that result in anasymmetric lift distribution.

DETAILED DESCRIPTION

Certain exemplary embodiments provide a means of solving a conundrum ofdesign for high altitude flight. Specifically, certain exemplaryembodiments comprise an aircraft that has a ceiling well into thestratosphere; the ability to loiter on station indefinitely at thatceiling; and structural robustness in the troposphere. Heretofore, nomore than two of these three conditions could be satisfied in oneaircraft design. The exemplary embodiments herein may provide theadditional benefits of operational flexibility and access to any pointon the globe within a matter of hours.

In a more general sense, the exemplary embodiments herein have thepotential to improve the endurance, range, robustness, and operationalflexibility of the most efficient of aircraft designs, the span-loadedflying wing. The exemplary embodiments herein may free the aircraftdesigner from constraints associated with span-wise bending, whetherresulting from gust loads or maneuvering loads. This is achieved bytrading structural benefits against the cost of additional complexity offlight control.

In a space exploration role, certain exemplary embodiments provide ameans of exploring planetary atmospheres. For example, certain exemplaryembodiments are well-suited for flight at low altitudes on Mars. In thegeneral case, benefits for flight in all planetary atmospheres, eventhose of high density may be realized. A modular aspect of certainexample embodiments greatly facilitates design of the spacecraft thatwould carry the aircraft across interplanetary space and then insert itinto a given planetary atmosphere.

In addition to freeing the aircraft designer from structuralconstraints, certain exemplary embodiments free the mission payloaddesigner from certain constraints of systems integration. Forsurveillance missions, sensor resolution increases with sensor size, orin the case of a sparsely populated phased array, with the length of thebaseline between the most widely separated elements. Certain exemplaryembodiments theoretically are unlimited in wingspan, so remarkably goodsensor resolutions may become possible. For communications payloads,large wingspan enables wide separation of antenna elements, whichreduces mutual interference and facilitates simultaneous operation ofmultiple receivers and transmitters.

Certain example embodiments relate to a compound aircraft systemcomprised of multiple autonomous modular elements that are joinedwingtip-to-wingtip. The compound aircraft is herein designated a“skybase” and its modular elements are designated “flyers.” The flyerstogether form a single, multiple-articulated flying surface of greataerodynamic efficiency. The skybase is a machine analog to a biologicalcolony or superorganism. The flyers when flying independently form aswarm. The swarm of flyers coalesces into a skybase with a differentmorphology, and the aerodynamic performance of the system is increasedin consequence. The skybase can aggregate or disaggregate at will, suchbeing the source of its aero-structural advantages and operationalflexibility.

FIG. 1 shows an exemplary multiple-articulated flying system having ahigh aspect ratio wing platform, and an enlarged view of an exemplaryflyer comprising such a system. Flyers 102 a-g are joinedwingtip-to-wingtip by a quick-connect and quick-release hinge mechanismto form skybase 100. This mechanism allows flyers to flap about thehinge with respect to each other. The mechanism also allows rotation inthe pitch direction (i.e. about the lateral axis). According to certainexemplary embodiments, there is no degree of freedom in yaw because of alack of need for one, however the present invention is not so limited.

A conventional wing is designed to resist span-wise bending moments thatresult from normal loads, whether caused by maneuvering or gusts. Thissets a floor for the structural strength and thus a weight of the wingstructure. In contrast, the articulated wing of a skybase is designed tobreak apart at some low threshold of loading—e.g. wingtip hinges serveas fusible links in a structural sense. After the high load event, theflyers reconnect and reform the skybase. Individual flyers haverelatively low aspect ratios and are quite stiff in span-wise bending.The skybase has a high aspect ratio but can be much lighter than anon-articulated wing, since it does not need to resist span-wise bendingmoments.

The wingtip hinges are designed to accommodate some degree of flappingbefore a loading event causes separation, so as to prevent nuisancedisaggregation. The flap limit is determined by the geometricconstraints of the gap seals that are necessary for aerodynamicperformance in the joined state. These gap seals cover the hinges onboth the lower and upper wing surfaces.

Wingtip hinge freedom in pitch gives the skybase a unique capability totailor its span-wise wing twist to operating conditions. For example,progressively reducing the angle of incidence of flyers as a function oftheir distance from the skybase centerline provides washout. Washout canbe used to modify the span-wise lift distribution to reduce lift-induceddrag. Washout also reduces the propensity of the inboard flyer of askybase to stall when the skybase is turning. This allows the skybaseturn rates to be greater than would be possible with a conventional wingof equivalent span. It will be appreciated that only a small range ofpitch variability is required to provide these benefits, and this rangemay be accommodated by the design of the hinge gap seals.

A skybase has an alternative method of turning that may be operationallypreferable. The individual flyers forming the skybase can separate, turnas individuals, and reform into a skybase that is oriented to a newheading. This method is analogous to the turning of a flock of birds. Itcircumvents the problem noted above, that the inboard flyer of a skybasehas a propensity to stall when the skybase is turning as a singleassembly.

FIG. 2 is an exemplary lifecycle of a skybase. The life cycle of askybase can be divided into seven phases: (1) a deployment phase (S202),(2) a launch phase (S204), (3) an assembly phase (S206), (4) a loiterphase (S208), (5) an optional disaggregation—detachment—reaggregationphase (S210), (6) a disaggregation phase (S212), and (7) a recoveryphase (S214). It will be appreciated that when the skybase is being usedin the space exploration role, phases (2), (6) and (7) may not exist.

Phase (1) involves deployment of the skybase to a theater of operationsthat may be on the other side of the globe, or in the case of spaceexploration, on another planet. In the terrene case, skybase flyers canbe sized for packaging into standard cargo containers, which are carriedby container ships to overseas ports where they are transferred totrucks. The trucks then carry the containers to forward operatinglocations, where the flyers are removed from the containers and preparedfor launch. In the military case, this method of deployment facilitatescovert prepositioning of skybase flyers and associated support hardware,since the cargo containers are outwardly generic.

An alternative deployment method applicable to military use allowsaccess to any point on the globe within a matter of hours. Flyers areloaded into one or more cargo aircraft, the number of flyers in eachaircraft being dependent on the size of cargo bay. The cargo aircraftfly to a theater of operations, and their cargo ramps are lowered inflight. Skybase flyers are then sequentially pulled from the cargoaircraft by means of a parachute extraction system. In this manner,multiple skybase flyers are delivered to an airborne assembly location,and phase (2), the launch phase, can be dispensed with. This method ofdeployment is also potentially covert, since the contents of the cargoaircraft will not be outwardly discernable, and the flyers can beair-launched beyond the range of ground-based detection systems.Finally, returning to the use of a skybase in space exploration, in oneexemplary embodiment, flyers can be packaged into a probe shaped as afrustum, either radially or circumferentially disposed but in both casesstanding on their wingtips. Below the frustum is a heat shield forplanetary entry. A parachute package and other ancillary equipment arelocated above the frustum. This probe is injected into a planetaryatmosphere, it decelerates, the heat shield is released and theparachutes deploy. Once a sufficiently low sink rate has been achieved,skybase flyers rotate out of the frustum, pivoting about the wingtips onwhich they have been resting. Once clear of the frustum, the flyersprogress to phase (3), the assembly phase.

In phase (2), the launch phase, flyers take off from an airfield, eitherindividually or in flights of multiple units depending on the width ofthe runway and on any operational need for a rapid launch process. In apreferred embodiment, the flyers have no landing gear but are equippedwith skids faired into the underside of their fuselages. The flyers maybe dolly-launched. A dolly is a wheeled cart that may or may not havemotive power. An individual flyer is placed on a dolly and disposed atthe end of a runway for launch. The dolly accelerates, either under itsown power or motivated by the flyer's propulsion system, and once theassembly reaches flyer takeoff speed the dolly releases the flyer whichthen promptly ascends. The dolly then decelerates and is returned to itsstart point for another launch. In this manner, the flyer does not haveto carry the dead weight of landing gear aloft.

FIG. 3 shows an exemplary deployment process. In step S302, a drogueextracts a flyer for an airdrop, allowing rapid deployment. A main chutedeploys from the flyer in step S304. Transport struts (optionally usedto avoid damage to the flyers during transport and chute deployment) arejettisoned in step S306. Tail and side force controllers deploy in stepS308 enabling the flyer to take flight. In step S310, flyers rendezvousto form a pair or doublet. The assembly process is described in furtherdetail below.

In phase (3), the flyers self-assemble, as shown in FIGS. 4A-4C. Flyers102 a-d locate each other, rendezvous, and connect in doublets. Thesedoublets then rendezvous with each other, and connect in quads, a quadmay join a doublet to form a hexad, and so on until the desired skybaseconfiguration has been achieved. Each time the number of flyers in askybase subassembly increases, its aspect ratio increases and as aconsequence, its ceiling increases. The tendency is thus for couplingsof larger numbers of flyers to occur at sequentially higher altitudes.This is generally optimum in a structural sense. Each flyer isstructurally robust, being of low aspect ratio design, and is thuswell-matched to high gust loads associated with the denser air at loweraltitudes in the troposphere. While gusts do occur at higher altitudes,they are mostly associated with convective or mountain wave activity,and can usually be avoided. To achieve high altitudes, a skybase mustcoalesce into a high aspect ratio platform that is structurally weaker.A conventional wing would break when encountering a gust at thesealtitudes, whereas a skybase may subdivide and then reform.

In principle, there is no limit to the number of flyers that can beconnected to form a skybase. In practice, however, there are diminishingreturns to aspect ratio in terms of aerodynamic efficiency. At theextreme, difficulties will be experienced when the skybase is wideenough to span a shear between air masses moving at differingvelocities, an event that would demand that the skybase spontaneouslysubdivide at the locus of shear. Such an event is herein defined as a“span shear.”

Thus, FIG. 4A shows a swarm of flyers 102 a-d locating each other. Theyrendezvous in FIG. 4B, and two doublets (102 a-b and 102 c-d) areformed. Finally, in FIG. 4C, the two doublets rendezvous to form skybase100.

In phase (4), the skybase loiters at altitude. Occasionally the skybasemay subdivide and reform when encountering gusts. If the skybase isflying into a headwind, it may match its airspeed to oppose the wind,and the skybase will then have zero groundspeed—it will be able to hoverover a point on the ground. If there is little or no headwind, theskybase must fly in orbits. These orbits could involve shallow turnsdescribing large circles over the ground, or they could be shaped asnarrow ovals, with the skybase disaggregating, flyers turning andreaggregating at each end of the oval, as previously described. It willbe appreciated that a preferred orbit shape would depend on therequirements of the particular mission payload that is carried by theskybase.

In phase (4), and indeed in all phases, the modular nature of theskybase results in very graceful degradation in case of loss of a flyer,or indeed in case of any subsystem failure within a flyer. Skybasesystem survivability and reliability is decoupled from that of itscomponent flyers. If flyers are lost during the assembly phase (e.g.from flying through a storm), a skybase can still be formed, albeit witha lower operational ceiling. In the military case, if a skybase isattacked by a surface-to-air weapon and one or more of its componentflyers are damaged, the skybase can disaggregate, reject its uselesscomponents, and reaggregate in abbreviated form. System capability wouldbe lost, but system functionality would be retained. Eventually,replacement flyers could be flown out to the skybase and full systemcapability would be restored. In a preferred embodiment, this capabilityto reconstitute a skybase gives it a capability for being “foreveron-station” that cannot be matched by any unitary (e.g. non-modular)design.

FIGS. 5A-5C show an exemplary reaggregation process. FIG. 5A showsskybase 100, including flyers 102 a-g. In FIG. 5B, flyer 102 c crashes(e.g. fails, is shot down, etc.). It will be appreciated that otherreasons for a flyer leaving a skybase may exist, such as, for example, aflyer being recalled, instructed to survey an area of interest from acloser location, etc. This leaves two smaller skybases, 100′ and 100″.FIG. 5C illustrates flyers 102 a-b and 102 d-g to form skybase 100″′,thus reaggregating into a fully-functional skybase, demonstrating systemsurvivability.

Phase (5) is an optional disaggregation—detachment—reaggregation phase.A skybase is in a sense a virtual aircraft carrier in the sky, a base ofoperations. Its modular design supports applications where it becomesnecessary to investigate locations that are removed from the primarymission station. In surveillance applications, a skybase detachment(e.g. a doublet) can be separated from the main skybase. The detachmentcan then proceed to a remote mission station, perform surveillance asrequired, and then return and reattach to the skybase. Alternatively, ifthe skybase finds itself temporarily above cloud cover that interfereswith its surveillance sensors, the detachment can descend to an altitudebeneath the clouds and ensure that surveillance is not interrupted.Finally, a detachment may be sent to a lower altitude simply to get ahigher resolution view of a target; a detachment may be offset from theskybase to peer behind a mountain range; it may be offset to enablebistatic sensor operation (e.g. having transmitters and receivers onspatially separated platforms); or, in the signals intelligence role, itmay be offset to enable triangulation for geo-location ofelectromagnetic emissions.

Phase (6) is the disaggregation phase, which occurs if recovery of acomplete skybase is desired. In contrast to the aggregation phase, therewould normally be no need to disaggregate the skybase until it hasdescended to a pattern altitude near the recovery base. It will beappreciated that disagreggation at high altitude may be preferred if itwere desired to disperse flyers to more than one recovery base in atheater of operations. This might be necessary, for example, to maintaina balanced force structure if some recovery bases had suffered highattrition of flyers from enemy action or adverse weather.

Phase (7) is the recovery phase. Flyers are sequenced into the landingpattern, and land individually or in flights. In a preferred embodiment,landing skids that were faired into the fuselage for takeoff areextended. Each landing skid has a pair of small wheels affixed to eitherside, and these wheels allow the flyer to taxi off the runway under itsown power, clearing the runway for subsequent flyers to land.

A preferred embodiment of the current invention is an unmannedsolar-electric modular articulated-wing aircraft. This aircraft is ableto reach the upper reaches of the Earth's atmosphere by virtue ofself-assembly of modular elements (“flyers”) at progressively higheraltitudes. As flyers are added, the aspect ratio and thus lift-to-dragratio of the articulated-wing aircraft (“skybase”) is increased.

An alternative embodiment differing only in size from the preferredembodiment would be suitable for operation in the Martian atmosphere atlow altitudes above ground level.

The skybase is able to loiter indefinitely at high altitude, eitherremaining fixed with respect to a location on the ground or performingturns about that location, depending on wind conditions. Aircraftendurance is not limited by fuel, since all power requirements aresatisfied by insolation—that is, irradiance by solar flux. Aircraftendurance is not limited by component failure, since additional flyerscan be flown out to the skybase to replace failed flyers as required.

The number of flyers required in a skybase is a function of severalvariables. For a given amount of electrical power supplied to themission payload, the greater the number of flyers, the higher thealtitude that can be achieved. Alternatively, at a given altitude, thegreater the number of flyers, the greater the electrical power that canbe supplied to the mission payload. Either way, the skybase must achievean altitude where the winds are sufficiently light to allow permanentflight with power derived from insolation alone. If the mission payloadrequires some minimum line-of-sight to the horizon, the minimum requiredaltitude (and hence, number of flyers) may be greater than that imposedby insolation constraints.

A solar-electric power system may comprise, for example, a photovoltaicarray; an electronic controller to condition and manage the flow ofpower in the system; means to store electrical energy (such as, forexample, a battery); and one or more electric motors to provide motiveforce. To allow continuous and indefinite operation, the solar energyused by the propulsive and mission systems in daylight plus the excesssolar energy stored in the battery must be at least equal to theelectrical energy consumed by the propulsive and mission systems duringthe night. In an exemplary embodiment, the power provided by the batterymust be sufficient to allow the countering of headwinds at any altitude,with some margin for climbing and maneuvering of flyers during assemblyof the skybase.

These considerations place a premium upon efficiencies of mechanicalcomponents, on aerodynamic efficiencies, and on the minimization ofbattery and structural weight, all to the end of achieving an altitudewhere the winds are low enough to allow permanent flight with theinsolation available. The current invention enables this objective bymaximizing aerodynamic efficiency while minimizing structural weight.

A preferred embodiment is like any other solar-electric aircraft,inasmuch as its absolute ceiling is greater than the altitude at whichit enjoys maximum endurance. In the case of an exemplary embodiment withthe capability for “forever on-station” operation, the maximum altitudeat which flight can be sustained indefinitely is herein defined asequilibrium ceiling. Absolute ceiling is determined by the maximumpropulsive power available from the solar-electric system at a giveninstant. Equilibrium ceiling is the altitude where energy available frominsolation over the course of a day is exactly balanced by the energyexpended by the aircraft over the course of a complete day-night cycle.The aircraft can operate above equilibrium ceiling temporarily, but ifit is to maintain indefinite flight it must survive through thefollowing night until the next insolation. Consequently, it must balancethe lost energy by descending below equilibrium ceiling for a time.

Equilibrium ceiling is severely reduced in winter and at high latitudes,because of reduced insolation. If equilibrium ceiling is reduced to thepoint that the high winds commonly found at lower altitudes areencountered (e.g. in a jet stream) flight cannot be maintained. However,an exemplary embodiment has an inherent ability to minimize this loss ofequilibrium altitude that is not shared by the prior art. The process issomewhat counterintuitive. As there is less insolation under winter andhigh latitude conditions, there is less energy to be stored in thecourse of a day; consequently, less battery capacity is required. Anexemplary embodiment allows battery capacity to be tailored toinsolation. For example, flyers with lighter “winter-weight” batteriescan replace heavy-battery “summer-weight” flyers as the seasonsprogress. There still will be a loss of equilibrium altitude (or moreflyers will be required to maintain a given equilibrium altitude), butthe effect will be minimized. In contrast, the unitary designs in theprior art would either suffer a much larger loss in equilibrium altitude(assuming that they could even maintain indefinite flight in summer,equatorial conditions), or would be forced to return to base.

FIG. 6 shows solar flux in watts per square meter as a function ofseason and latitude at an altitude of 17 km (about 56,000 ft). Time ofday is shown on the horizontal axis. Incident flux at the wintersolstice (December 22 in the northern hemisphere) is much less thanduring the summer solstice (June 22 in the northern hemisphere),especially at high latitudes. Under conditions of low solar flux, askybase may either reduce altitude (unless that is prevented by thepresence of high winds at lower altitude), reduce the electrical load ofany mission payload, and/or increase the number of connected flyers. Thelast option is the great operational advantage of the modular skybasedesign—e.g. the ability to tailor aircraft size to the available solarflux.

FIGS. 7A-7F show skybase equilibrium ceilings as a function of thenumber of connected flyers at various latitudes and at various times ofthe year. Specifically, FIG. 7A presents the equilibrium ceiling of askybase as a function of the number of connected flyers, for flight at45 degrees of latitude at the summer solstice. Referring to the FIG. 6showing solar flux, it can be seen that this represents a favorable,although not the best, case for solar flight. For low equilibriumceilings (e.g. low numbers of connected flyers), it is assumed thatthere is no obscuration of solar flux as a result of cloud cover. Theupper curve shows equilibrium ceiling for a skybase with no electricalload from a mission payload. The lower curve shows the reduction inequilibrium ceiling that results from an electrical draw of 143 wattsper flyer (equating to one kilowatt in the seven-flyer skybase). Thesecurves are valid for the parametric assumptions listed in Table 1, whereη refers to efficiency, coverage factor refers to the proportion of wingarea covered by photovoltaics, E refers to energy, W_(e) is emptyweight, W_(batt) is battery weight, W_(o) is total weight, C_(lmax) ismaximum lift coefficient, and V_(cruise) is cruise velocity. Batteriesare sized to the available solar flux, which is high—hence, these are“summer-weight” batteries.

TABLE 1 Parametric Assumption Value Propeller η 0.85 Motor η 0.89Battery η 0.83 Solar cell η 0.30 Coverage factor 0.92 Battery duration12 hrs. Battery E density 350 Whr/kg Struct. wing loading 0.8 lb/ft²W_(e)/flyer 166 lbs. W_(batt)/flyer 176 lbs. W_(o)/flyer 342 lbs. Wingarea/flyer 208 ft² C_(Lmax) 1.4 V_(cruise) 33-129 KTAS

For this condition, only three flyers are necessary to achieve a“forever on-station” altitude of 60,000 feet. At this altitude or above,it is reasonably certain that low winds will prevail. It will beappreciated that for this case, equilibrium ceiling is relativelyinsensitive to electrical demands from a mission payload.

FIG. 7B shows equilibrium ceiling for the midwinter case at a latitudeof 36 degrees. Battery weight is scaled to the available sunlight—hence,each battery pack weighs 80 pounds instead of 176 pounds in the previouscase. These are “winter-weight” batteries. It will be appreciated thatit now takes more flyers to achieve a “forever on-station” altitude of60,000 feet. It also will be appreciated that the effect of a missionelectrical draw is relatively more severe. Table 2 lists the parametricassumptions.

TABLE 2 Parametric Assumption Value Propeller η 0.85 Motor η 0.89Battery η 0.83 Solar cell η 0.30 Coverage factor 0.92 Battery duration15.5 hrs. Battery E density 350 Whr/kg Struct. wing loading 0.8 lb/ft²W_(e)/flyer 166 lbs. W_(batt)/flyer 80 lbs. W_(o)/flyer 246 lbs. Wingarea/flyer 208 ft² C_(Lmax) 1.4 V_(cruise) 20-63 KTAS

FIG. 7C shows the effect of an increase in latitude to 45 degrees, stillmidwinter with winter-weight batteries. This is a challenging case. Withten or fewer flyers, it is no longer possible for a skybase to achievean equilibrium ceiling of 60,000 feet. Unless winds at lower altitudeare low, “forever on-station” flight will not be possible. This showshow solar flux restricts the operation of even the most efficient solaraircraft. Table 3 lists the parametric assumptions

TABLE 3 Parametric Assumption Value Propeller η 0.85 Motor η 0.89Battery η 0.83 Solar cell η 0.30 Coverage factor 0.92 Battery duration16.5 hrs. Battery E density 350 Whr/kg Struct. wing loading 0.8 lb/ft²W_(e)/flyer 166 lbs. W_(batt)/flyer 52 lbs. W_(o)/flyer 219 lbs. Wingarea/flyer 208 ft² C_(Lmax) 1.4 V_(cruise) 16-44 KTAS

FIG. 7D shows the crippling effect of flying in winter with batteriesthat are sized for summertime levels of solar flux. Equilibrium ceilingsare below ground level for less than five-flyer or nine-flyer skybases,depending on mission electrical demands. In such cases, perpetual flightcannot be maintained at any altitude, whatever the prevailing winds.This demonstrates a great advantage of the modular skybase design. Sincewinter-weight flyers can cycle out to a skybase to replace summer-weightflyers as the seasons progress, gross battery weight can be continuouslytailored to available solar flux, and flight performance can thereby bemaximized. This is not possible with a unitary (e.g. non-modular)design. Table 4 lists the parametric assumptions.

TABLE 4 Parametric Assumption Value Propeller η 0.85 Motor η 0.89Battery η 0.83 Solar cell η 0.30 Coverage factor 0.92 Battery duration55 hrs. Battery E density 350 Whr/kg Struct. wing loading 0.8 lb/ft²W_(e)/flyer 166 lbs. W_(batt)/flyer 176 lbs. W_(o)/flyer 342 lbs. Wingarea/flyer 208 ft² C_(Lmax) 1.4 V_(cruise) 16-28 KTAS

FIG. 7E shows the sensitivity of the skybase design to the parametricassumptions of the previous cases. The upper curve represents the samecase as the 143 watts per flyer electrical load case of FIG. 7A. Thelower curve shows the effect of a less-challenging set of designcriteria. It can be seen that one more flyer (i.e. a total of four) isrequired to achieve an equilibrium ceiling of 60,000 feet. Table 5 liststhe parametric assumptions corresponding to less challenging criteria.

TABLE 5 Parametric Assumption Value Struct. wing loading 0.85 lb/ft²W_(e)/flyer 177 lbs. W_(batt)/flyer 176 lbs. W_(o)/flyer 353 lbs. EnergyDensity 300 Whr/kg Solar Cell η 0.27 Coverage Factor 0.88

FIG. 7F parallels the circumstances of FIG. 7C—a midwinter skybase at 45degrees latitude with winter-weight batteries, again with 143 watts perflyer electrical load. It will be appreciated that the loss ofequilibrium ceiling that results from relaxed design criteria is moresevere than in the previous case. Table 6 lists the parametricassumptions corresponding to less challenging criteria.

TABLE 6 Parametric Assumption Value Struct. wing loading 0.85 lb/ft²W_(e)/flyer 177 lbs. W_(batt)/flyer 53 lbs. W_(o)/flyer 229 lbs. EnergyDensity 300 Whr/kg Solar Cell η 0.27 Coverage Factor 0.88

FIG. 8A is a plot of mean wind speed as a function of altitude andlatitude for the case of a northern hemisphere winter. It is importantto note the location of the northerly and southerly jetstream cores, andhow the winter jetstream is stronger than the summer jetstream. It alsois important to note that there is wide variability in wind speeds inthe upper atmosphere from day to day that is not captured by this plot.Nevertheless, it is a useful tool for visualization of mean windconditions.

FIG. 8B superimposes a representation of estimated skybase performanceon the mean wind plot from FIG. 8A. The sun is shown over the Tropic ofCapricorn, corresponding to the summer solstice in the southernhemisphere. It is assumed that flyers are added as required, up to alimit of ten. An electrical load of 143 watts per flyer is assumed.Lighter areas designate combinations of altitude and latitude where askybase can operate indefinitely. Darker areas designate regions wherewinds are too high for a skybase to maintain position over a fixedlocation on the ground, even though equilibrium ceiling considerationswould otherwise enable perpetual flight. The upper bound of both lighterand darker areas represents equilibrium ceiling. A skybase can operateabove this contour, but only for limited periods. Generally, no flightis possible north of the Arctic circle. This is logical, as there isconstant darkness there in this case of a northern midwinter. Incontrast, Antarctica is at the same time the land of the midnight sun.Solar-powered aircraft can fly, but the sun is very low to the horizon,and the glancing incidence of sunlight to the photovoltaics reducesefficiency and thus altitude performance.

It will be appreciated that in the northern winter in mean windconditions, it is possible for a skybase to maintain flight over alocation on the ground indefinitely, up to a latitude of perhaps 45degrees.

FIG. 9 shows skybase airspeed superimposed on winds aloft near Baghdad,Iraq, observed between 1958 and 1990. It includes unclassifiedinformation from military weather records provided by the United StatesAir Force Combat Climatology Center, Asheville, N.C. The solid dark linerepresents mean winds. The light-shaded region represents the variationof observed winds. An electrical demand of 143 watts per flyer isassumed. The number of flyers in a skybase is allowed to vary. Theleft-hand boundary of the dark-shaded region depicts skybase airspeed inmidwinter, while the right-hand boundary represents midsummer. “Foreveron-station” flight is achieved where the dark-shaded region departs fromthe light-shaded region—in this case, at about 57,000 feet. Inmidwinter, this necessitates nine flyers in the skybase. In midsummer,three flyers generally are required.

FIG. 10 illustrates a preferred arrangement of a skybase flyer. Theflyer is of conventional configuration, with wings, fuselage, propeller,and empennage. The entirety of the upper wing surface 1 is covered withan embedded and laminar photovoltaic array. The lower wing surface 2forms the mounting surface for an antenna array, either forcommunications, surveillance, signals intelligence, or the like.

A preferred embodiment of the current invention incorporates asurveillance sensor (not shown). A surveillance sensor for a span-loadedaircraft is itself ideally distributed across the wingspan. This avoidshigh point loads that could lead to catastrophic structural failure.Furthermore, in the solar-electric case of a preferred embodiment, verylittle electrical power is available for the sensor. This results in thefavoring of passive as opposed to active sensors, for example usingpassive radiometry. It is also possible to incorporate a dual-modepassive sensor system, such as one using both radiometers (which canpenetrate clouds) and combined electro-optical and infrared sensors(which cannot, but which have higher sensitivities than radiometers).

In a preferred embodiment, the surveillance sensor integrated into eachflyer is comprised of an electronically-scanned antenna array installedin the lower surface of the wing 2, along with associated processinghardware. As an augmentation in an alternative embodiment, a lightweightelectro-optical and infrared sensor can be installed in the fuselage 3of each flyer. The processing architecture is decentralized to themaximum practical extent, to minimize weight concentrations and to easecooling issues such as may exist. Each flyer has an individualcapability to form an electronic beam, allowing image formation to aresolution limit imposed by the wingspan of the flyer (and, to a lesserextent, the overall length of the flyer). Images may be formedinterferometrically, if sparse arrays are called for by dint of theweight and power limitations of a preferred embodiment, or may rely onfully-populated phased arrays. Antenna elements can be placed, forexample, along the tail boom 7 of each flyer to extend the length ofeach array; may be placed at the end of extensible poles mounted in a“stinger” position; or may be towed on a drogue behind the flyer.

To enable image forming, a real-time calibration of the antenna array isperformed, so as to compensate for the effects of wing flexing or drogueposition errors. This is done using the techniques of opticalinterferometry, either on fiber optics placed inside the aircraftstructure, or using lasers in free-space.

In forming a skybase, each flyer performs a rendezvous and docks withone or more other flyers. The architecture of the sensor system is suchthat, on docking, the individual antenna arrays are linked in functionto enable image forming at higher resolution, taking advantage of theincreased wingspan. As the ever-larger skybase ascends to itsprogressively-increasing equilibrium altitude, resolution at the groundtarget is maintained (or even increased) by virtue of the larger antennasize. Conversely, if individual flyers separate from the skybase, thediminished skybase normally descends to a lower equilibrium altitude andsensor angular resolution is reduced (due to its now-shorterwingspan)—but resolution at the ground is maintained due to the lowerline-of-sight slant range to the target. Meanwhile, the detached flyeror flyers can proceed under their own power to a remote operatinglocation, at a lower altitude and perhaps under a cloud cover, andretain the capability to image targets with their individual sensorarrays.

Skybase flyers communicate with each other while separated via awireless intranet, but communication may be limited to upper-levelfunctionality (system status, relative position, and task allocation)because of bandwidth constraints. In certain example embodiments, thereis no need to pass any level of processed imagery between flyers.

When compounded into a single, articulated structure, linked flyers cancommunicate with each other via fiber-optics. Inter-flyer optical datatransfer can be conducted by locating lenses at the flyer wingtips, at aterminus of each flyer's fiber-optical data bus. When flyers areconnected, lenses oppose lenses, wingtip to wingtip, and optical data ispassed across a few inches of free-space. Consequently, flyers can docktogether while avoiding the need for mechanical data businterconnections and their associated complexity. In this case, veryhigh bandwidth is possible, and image formation is enabled using theentire span of the skybase, however many flyers are connected.

Returning to FIG. 10, the flyer fuselage 3 is suspended from the flyerwing by a pylon 4. Location of the fuselage below the wing serves toprovide a natural restoring force that opposes flapping motions of thoseflyers at the outboard stations of the skybase, thereby relievingrequirements levied on any anti-flapping modes of the skybase flightcontrol system. A suspended fuselage location also facilitates launchand recovery of the flyer from an airfield, elevating the wing off theground, and allowing some banking into a crosswind if necessary withoutfear of catching the wing and cart-wheeling. The underside of thefuselage has hard points fore and aft to allow the flyer to rest in alaunch dolly. A retractable skid is faired into the underside of thefuselage for airfield landings. A small wheel for taxiing purposes islocated either side of the skid. Within the fuselage 3 and pylon 4 arelocated a battery pack (either summer-weight or winter-weight); anelectronic controller module to provide energy conditioning andmanagement; an electric motor for propulsion; a reduction gearbox drivenby the motor that drives the propeller 5; a flight avionics packageincluding communications and navigation equipment and a flight controlsystem; and a mission avionics package containing a central processorfor the antenna array fitted to the undersurface of the wing 2. In analternative embodiment, an electro-optical and infrared surveillancesensor comprising a camera and turret is also fitted in the fuselage 3.

A vee-tail empennage (tail 6) is attached to each flyer by a tail boom7. This tail boom extends from a center-wing fairing 8. A preferredembodiment of the flyer design includes a tail surface to improvecontrol authority in pitch of each flyer when flying as a single unit inthe lower atmosphere. The tail also provides a pitch control surfacethat is decoupled from flapping motion when the flyer is a connectedelement of a skybase. The design of the tail collapses for shipment ofeach flyer in a standard cargo container or in the cargo bay of atransport aircraft. The sequence is as follows: tail 6 folds flat,forming a surface parallel to the upper wing surface 1. Tail 6 and tailboom 7 then slide as a unit into center-wing fairing 8. In stowedconfiguration, the trailing edge of tail 6 is coincident with thetrailing edge of the upper wing surface 1, as can be seen in FIG. 12.

The center-wing fairing 8 also includes an attachment point for aparachute system. This parachute system comprises a sequentially-openeddrogue parachute and main parachute, and is used for extraction of theflyer from a transport aircraft, in the case of air-deployment aspreviously described.

Inboard flaperons 9 and outboard flaperons 10 provide control authorityin roll (motion about the longitudinal axis) for a flyer operating as anindividual unit. For landing and takeoff, flaperons 9 are used asconventional flaps, and flaperons 10 are used as conventional ailerons.When the flyer is connected to other flyers, the function of thesecontrol surfaces changes. Flaperon actuation must be coordinated betweenflyers, using their networked flight control systems. The flaperons 9and 10 become flapping-dampers, serving to damp flapping motions betweenadjacent flyers. When flaperons 9 are deflected in opposition toflaperons 10 across a semi-span of the skybase, a yawing moment isgenerated about the vertical axis of the skybase. When this action iscombined with a downward deflection of the elevator at the trailing edgeof the tail 6 of each flyer across that same semi-span, the entireskybase performs a coordinated turn in that direction. The elevators ofthe flyers become elevons when the flyers are linked into a skybase.

Suspended beneath each flyer wing is a retractable side-force controller11. The side-force controllers 11 are located at the fore-and-aftlocation of the flyer center of gravity, and thus are capable ofimparting force along the lateral axis of the flyer without thegeneration of any unwanted yawing moment. The side-force controllers 11impart this force by rotation about their vertical axes. The purpose ofthese control surfaces is to allow flyers to rendezvous and dock in assimple a manner as possible. Conventional aircraft control surfaceswould demand that the flyers close intervening gaps by rolling towardseach other by aileron control, or alternatively, skidding towards eachother with rudder control. In both cases, rotational motions would bedeveloped that are not conducive to a rendezvous and mechanical docking.Provision of side-force controllers 11 enables a direct lateraltranslation that is not otherwise possible. Direct lateral translationminimizes the chance that one flyer could blunder into the trailingwingtip vortices generated by the other during a tip-docking maneuver.

Side-force controllers 11 are mounted to the lower wing surface 2 sincemounting on the upper wing surface 1 would tend to reduce the wingsurface available to the photovoltaic array. Mounted beneath the wing,they can also double as skids to protect the lower wing surface 2 duringlanding. They are retractable into the lower wing surface 2 in order toreduce aerodynamic drag when not in use.

FIG. 10 depicts a front ball 12 and a rear ball 13. These balls arefitted to the starboard flyer wingtips in the preferred embodiment, andare part of the inter-flyer connection and articulation mechanism thatis fully illustrated by FIG. 13.

A reliable method of aggregating flyers into a skybase is crucial froman operational perspective. The rendezvous and docking system may needto function in turbulent air, at any time of day or night. The preferredembodiment of the current invention involves a flyer performing arendezvous to a line abreast pre-docking position. Gross navigation tothis position is performed by a pseudolite-based Global PositioningSystem receiver and transmitter set. The final docking maneuver isconducted with the aid of side-force controllers 11 as described above.The relative position of one flyer with respect to another in the finaldocking phase is determined by a short-range ladar system. In turbulentair, however, it may be difficult to reliably tip-dock flyers, and amore robust means of closing the gap may be required. FIG. 11 depicts analternative embodiment of the skybase flyer design, showing mechanicaland aerodynamic aids to tip-docking. FIG. 11 depicts a handedness thatcan be reversed in an alternative embodiment without any effect on fitor function. In a preferred embodiment, a boom 14 telescopes out from aflyer's starboard wing. A controllable drogue 16 is paid out on a line15 passing over a small pulley at the tip of the boom 14. A second flyerapproaches the first flyer from behind, offset so as to line up anelectromagnetic receiver pad on its port wing behind the drogue 16 beingtrailed by the first flyer. The typical location of the receiver pad isshown in 17. The drogue 16 makes contact with the receiver pad,whereupon an electric current is applied to the pad so as to capture thedrogue 16 electromagnetically. At that point, the line 15 is hauled inas the two flyers adjust their airspeeds to provide a closing velocity.Eventually the two flyers are in line abreast, and the first flyer'sboom 14 locks into a slot in the forward port wingtip of the secondflyer. The boom 14 is then retracted into the wing of the first flyer,guiding the wingtips of the two flyers together for a final connection(depicted in FIG. 13 below).

A possible disadvantage of the alternative embodiment described above isthat one flyer must trail the other, albeit to one side. It isconceivable that in the pre-docking positioning, the trailing flyercould blunder into the trailing vortex being shed by the wingtip of thelead flyer, and suffer a roll upset that would be difficult to recoverfrom without the loss of significant altitude. Vortex controllers 18 areconsequently fitted to the wingtips of each flyer in an alternativeembodiment. These aerodynamic surfaces have a limited capability toinfluence the lateral spreading and core strength of the trailing tipvortices.

FIG. 12 shows the arrangement of two flyers packaged into a standard40-foot shipping container 20. The empennage 6 and tail boom 7 of eachflyer are shown in the stowed state—that is, with empennage 6 foldedflat and tail boom 7 retracted into center-wing fairing 8. A flyerpropeller 5 is shown with its spinner removed for shipping. The flyersare shown with transport struts 19 installed. These struts preventdamage to the relatively fragile flyers during shipping, and are removedwhen the flyers are prepared for flight at a launch base. If a flyer isto be air-launched by parachute extraction from a transport aircraft,the transport struts 19 are left in place to be jettisoned once theflyer is pulled clear and is suspended from its main parachute. Thisensures that the fragile flyer survives the mechanical shock associatedwith parachute extraction.

FIG. 13 illustrates a preferred embodiment of the skybase connection andarticulation mechanism. This figure depicts a handedness that can bereversed in an alternative embodiment without any effect on fit orfunction. The connection and articulation mechanism is very simple andlightweight compared to the equivalent mechanisms found in the priorart, such as those designed for use in Projects Tip Tow and Tom Tom inthe 1950s. This is because the skybase flyers are themselves light and,as a result, correspondingly low forces are applied to the mechanismduring docking and connected flight.

In an exemplary embodiment, as two flyers commence their final dockingsequence, a hinge carrier 21 slides out from the starboard wingtip ofone flyer in the direction A. A front ball 12 and a rear ball 13 areincorporated in the hinge carrier 21. These balls have vertical freedomof movement while otherwise restrained in a forward vertical slot 24 anda rear vertical slot 25. As the port wingtip of a second flyerapproaches the starboard wingtip of the first in the direction B, afront socket 22 and a rear socket 23 extend from the aforesaid portwingtip, and open so as to receive ball 12 and ball 13. The balls andsockets make contact, and the sockets 22 and 23 are commanded to theclosed position. At this point, the hinge carrier 21 is retracted backinto the wingtip of the first flyer. Finally, a gap seal 26 attached tothe upper surface of the port wing 1 of the second flyer is extended.The gap seal 26 traverses along trackways 29. Once fully extended overthe now-connected articulation mechanism, the gap seal 26 is free toflex about its front hinge 27 and its rear hinge 28.

Once engaged, the inter-flyer connection mechanism accommodates aflapping motion about the longitudinal hinge axis. The sense of motiondepicted by C represents a wing droop. The connection mechanism alsoaccommodates variation in the relative pitch of flyers with respect totheir lateral axes. If the second flyer as shown is outboard of thefirst flyer as shown, and both are connected in a skybase assembly, adeflection in the direction D of the forward ball and socket jointcomprising 12 and 22, when combined with a deflection in the direction Eof the rear ball and socket joint comprising 13 and 23, results in ageometric washout of that semi-span of the skybase wing.

FIG. 14 shows the restoring moment acting about the flapping axis of twoconnected flyers where one flyer is at the outside of the skybase. Thefollowing equation represents the net moment:

${{Net}\mspace{14mu}{Moment}} = {{{{L( \frac{b}{2} )}( {\cos\;\theta} )} - \lbrack {{{W( \frac{b}{2} )}( {\cos\;\theta} )} - {{Wh}( {\sin\;\theta} )}} \rbrack} = {{Wh}( {\sin\;\theta} )}}$Here, L is lift, W is weight, b is the flyer wingspan, h is the verticalseparation of the fuselage from the wing, and θ is the flapping angle.It will be appreciated that the net restoring moment is linear with h,and so from this consideration at least, it is desirable to suspend theflyer's fuselage from its wing. This calculation assumes a symmetriclift distribution across the flyer's wing.

FIG. 15 shows actual outboard wingtip effects that result in anasymmetric lift distribution. In actuality, outboard wingtip effectswill result in an asymmetric lift distribution that will in turn cause astatic droop of the outboard flyer. This droop can be countered withflaperon deflection, although at the cost of some trim drag. Loweringthe flyer's center of gravity will also reduce static droop. However,the droop effect can be beneficial, since as a result, two flyers thatseparate will tend to fly apart rather than together, thus reducing thechance of a collision.

It is interesting to note that this flapping system is an undampedoscillator. It may be necessary to damp flapping motions with eitherflight control system inputs or by use of a mechanical damping device.The former may be preferable, since the latter would involve a weightpenalty.

It will be appreciated that the flapping hinge is not fixed in space.Each flyer's electric motor will impart a torque about the wingtip hingemechanism, and the effect will be most pronounced for the outboardflyers of a skybase. This is actually a positive feature, as motortorque variation can be used in concert with flight control surfaces tomanage the structural dynamics of the skybase assemblage.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. An autonomous modular flyer operable to loiter over an area ofinterest at a first high altitude, comprising: an airborne object havingtwo wings, each wing having a wingtip, the wingtips being operablyjoinable to at least one other autonomous modular flyer's wingtips toform an aggregation when a first predetermined condition is met, andbeing operably disaggregable from the at least one other autonomousmodular flyer's wingtips when a second predetermined condition is met;wherein each said autonomous modular flyer includes first and secondside-force controllers being respectively located on the bottom of theport and starboard wings, the side-force controllers being configured toimpart selective aerodynamic forces along a lateral axis of the flyerresulting in lateral translation during flyer aggregation, eachside-force controller being in line with the center of gravity of theflyer, wherein the aggregation forms a multiple-articulated flyingsystem having a high aspect ratio wing platform, operable to loiter overthe area of interest at an altitude at least as high as the first highaltitude, and wherein, when changing directions, the autonomous modularflyers in the aggregation are operable to disaggregate prior to, andre-aggregate once, each autonomous modular flyer in the aggregation hasindividually changed directions.
 2. The autonomous modular flyer ofclaim 1, being further operable to match its airspeed with a prevailingheadwind so as to loiter over the area of interest.
 3. The autonomousmodular flyer of claim 1, wherein the autonomous modular flyer has bothan altitude ceiling high in Earth's stratosphere and structuralrobustness in Earth's troposphere.
 4. The autonomous modular flyer ofclaim 1, further comprising a wingtip hinge on at least one wingtipallowing two operably joined modular flyers to flap about the wingtiphinge with respect to each other.
 5. The autonomous modular flyer ofclaim 1, wherein aggregations of larger numbers of modular flyers occurat sequentially higher altitudes.
 6. The autonomous modular flyer ofclaim 1, wherein the second predetermined condition includes one or moreof: a loading event above a given load threshold, a gust above a gustthreshold, a turn of the multiple-articulated flying system, a spanshear above a span shear threshold, an instruction for at least one ofthe modular flyers to undertake a remote surveillance activity, and aninstruction for at least one of the modular flyers to move closer to thearea of interest.
 7. The autonomous modular flyer of claim 1, whereinthe multiple-articulated flying surface of claim 1 is operable toreaggregate based at least on a third predetermined condition.
 8. Theautonomous modular flyer of claim 7, wherein the third predeterminedcondition includes one or more of: a second predetermined condition thatpreviously was met no longer is met, and at least one modular flyerbeing destroyed, recalled, and/or no longer functional.
 9. Theautonomous modular flyer of claim 1, further comprising insolationcircuitry to power each modular element and/or the multiple-articulatedflying system.
 10. The autonomous modular flyer of claim 9, wherein theinsolation circuitry comprises a photovoltaic array, an electroniccontroller to condition and manage the power, and an electrical energystorage mechanism.
 11. The autonomous modular flyer of claim 1, furthercomprising a flight controller operable to calculate an equilibriumceiling altitude and to instruct the autonomous modular flyer to climbor descend to the equilibrium ceiling altitude.
 12. The autonomousmodular flyer of claim 1, further comprising a sensor operable to gatherdata relating to the area of interest.
 13. The autonomous modular flyerof claim 12, wherein the sensor is further operable to work as anelement in a sensor array when a multiple-articulated flying system isformed, each element in the sensor array working in synchrony with eachother element in the sensor array.
 14. The autonomous modular flyer ofclaim 1, wherein each wingtip has a pitch freedom selected to enablespan-wise wing twist of the autonomous modular flyer to be changeablebased on operating conditions.
 15. The autonomous modular flyer of claim1, wherein the autonomous modular flyer is configured to store andoperate on different types and/or quantities of batteries, the typeand/or quantity of battery being selected based on seasonal changes, andwherein the autonomous modular flyer is further configured todisaggregate from an aggregation, if formed, to obtain a new batterypack, and then re-form the aggregation.
 16. A method of forming amultiple-articulated flying system having a high aspect ratio wingplatform, operable to loiter over an area of interest at a highaltitude, the method comprising: providing at least two autonomousmodular flyers, each having two wings with wingtips thereon; joining thewingtips of the at least two autonomous modular flyers when a firstpredetermined condition is met to form an aggregation; anddisaggregating the wingtips of the at least two autonomous modularflyers when a second predetermined condition is met, wherein each saidautonomous modular flyer includes first and second side-forcecontrollers being respectively located on the bottom of the port andstarboard wings, the side-force controllers being configured to impartselective aerodynamic forces along a lateral axis of the flyer resultingin lateral translation during flyer aggregation, each side-forcecontroller being in line with the center of gravity of the flyer, andwherein, when changing directions, the autonomous modular flyers in theaggregation are operable to disaggregate prior to, and re-aggregateonce, each autonomous modular flyer in the aggregation has individuallychanged directions.
 17. The method of claim 16, further comprisingmatching the multiple-articulated flying system's airspeed with aprevailing headwind and/or making large orbits in order to loiter overthe area of interest.
 18. The method of claim 16, further comprisingallowing joined wingtips to flap about wingtip hinges attached to thewingtips.
 19. The method of claim 16, further comprising re-aggregatingdisaggregated autonomous modular flyers.
 20. The method of claim 16,further comprising powering the autonomous modular flyers and/or themultiple-articulated flying system by using solar-electric energy. 21.The method of claim 16, further comprising calculating an equilibriumceiling altitude for the autonomous modular flyer, and altering theautonomous modular flyer's altitude to the equilibrium ceiling altitude.